This invention relates generally to VAR generators and it relates in particular to control systems for VAR generators which utilize time-related-error minimizers.
Static VAR generators compensate for the effects of highly reactive loads, such as arc furnaces, on the terminal variables of an AC supply system. The static VAR generator or flicker compensator, as it is sometimes called, includes a control circuit which measures the magnitudes of arc furnace currents in consecutive half cycles of the line or supply voltage. The measured information is used to compute necessary compensating currents in terms of required firing angle signal (.alpha.) for the thyristors of the static VAR generator. The static VAR generator is adjusted only once during each half cycle per phase. Generally, prior art concerning static VAR generators is directed towards apparatus and method for determining the proper firing angle for the inductor controlled thyristors of the VAR generator. The trend in the prior art is in a direction from apparatus associated with the determination of the firing angle by any means whatsoever towards apparatus for determining the firing angle at a point in time as close as possible to the actual firing of the thyristors. Examples of that trend can be found by examining the following U.S. Patent in sequence. U.S. Pat. No. 3,936,727, issued Feb. 3, 1976 to F. W. Kelly, Jr. and G. R. E. Lezan, teaches a compensation control device which determines firing time for a static switch in accordance with the magnitudes of the reactive load current and which additionally teaches a regulating means which controls the firing time to maintain the line current and line voltage at a selected line location in substantial phase coincidence. U.S. Pat. No. 3,999,117, issued Dec. 21, 1976 to Gyugyi et al, teaches a static VAR generator and compensator where time delayed firing angles are calculated by integrating furnace load currents over predescribed intervals during real time to thus maintain balanced load current at specified phase angles, which are usually zero for a three-phase electrical system. U.S. Pat. No. 4,000,455, issued Dec. 28, 1976 to Gyugyi et al, teaches that the total computation time for determining the firing angle in each succeeding half cycle will take no longer than the length of time of the last complete half cycle immediately prior to (measured at voltage peak) the firing of the thyristor. U.S. Pat. No. 4,068,159 issued Jan. 10, 1979 to L. Gyugyi teaches that the computation time for determining the firing angle can actually extend into the half cycle in which correction is to be applied. A copending application, Ser. No. 880,270 also by Gyugyi et al, teaches that the computation time for determining the firing angle is extended even further into the half cycle into which correction is to be applied. It has been noted, however, that the accurate, reliable, timely determination of firing angle from circuit variables becomes relatively less significant if the transport time or the delay time of the control system through which is information must be propagated is too long to allow the control system to respond to the frequency of the load disturbance for which the VAR generator is provided to correct. It follows, therefore, that the compensating current provided by the VAR generator tends to lag the load current which it is to compensate. This delay, which separates the load current demanded and the one actually provided by the VAR generator, introduces an error in load compensation, the magnitude of which increases with the frequency of load variation. As a consequence, the effectiveness of a VAR generator for load compensation can rapidly decrease as the frequency of load variation increases. It would be desirable, therefore, to provide a VAR generator control system where reduction in this error is provided.